As far as I know, there are no Pokémon that learn Giga Impact that do not also learn Return, which is just more damage per turn, plain and simple (and most learn Double-Edge as well, if you're finding that you just barely miss the OHKO on some Pokémon). Outside of Snorlax for its Z-move, Slaking, and some *very* niche Mega Salamence sets, there really is no reason to run Giga Impact over Return.